solar confusion

[matt195583]

OK I have been having a bit of trouble with some info on current draw and the like. A prime example is according to ohms law a 12 volt appliance that draws 12 watts draws 1 amp . a 240 volt appliance that draws 40 watts draws 0.16 amps ........ surely it cannot be more efficient to run a 240 volt appliance through an inverter , can it ? please if any one can help here it would be much appreciated .

[s/v necessity]

Power = Amperage x Voltage.    So *in a perfect world* it wouldn't matter.   IF the power consumption is the same, then as the voltage changes the amperage changes in accordance.  For example a 240 watt appliance running on 12 volts would consume 20 amps, a similar 240 watt appliance running at 120 volts would consume 2 amps, and running at 240 volts it would consume 1 amp.

    In the real world I think this mostly holds true, but there are some sticky details that can throw it off a bit.  For example I think higher voltages are generally more efficient.  Perhaps someone can elaborate.

   In reality you are using power (watts) not amps.  So if an inverter steps the voltage up, it will still need to draw the required ampsxvoltage from the battery bank.

[s/v Copacetic]

Don't forget that your inverter is not 100% efficient. I don't know what typical inverter efficiency is, but most have fans to dissipate heat. Heat is wasted energy (not to mention the fan  ;) )

[AdriftAtSea]

While higher voltage appliances may be a bit more efficient, since a boat generally only has a 12 VDC power system, it really isn't more efficient to run an appliance through an inverter, especially if you can run the appliance via 12 VDC directly.  The inverter generates heat, which is due to the fact that it is less than 100% efficient. The appliance, especially if it is a 12 VDC appliance to begin with, will also generate some heat in the voltage conversion process—wall wart, transformer, whatever.

As an example, if an appliance, say an Engel dual-voltage refrigerator, draws 3 amps at 12 VDC, it is a 36 WATT appliance.  If you were to run the same refrigerator at 120 VAC, it would only draw about .3 amps, since it is a 36 Watt Appliance (120 VAC * .3 Amps=36 watts).  However, the inverter would only be about 85% efficient, so the actual DC usage of the 36 watt appliance via the inverter would be 42.35 watts of power... (42.35 *.85 = 36 watts).

So, if you run the refrigerator off of 12 VDC, it consumes 36 watts per hour or uses 3 amp-hours per hour of use.  If you run it via the inverter, it consumes 42.35 watts or 3.53 amp-hours per hour. 

OK I have been having a bit of trouble with some info on current draw and the like. A prime example is according to ohms law a 12 volt appliance that draws 12 watts draws 1 amp . a 240 volt appliance that draws 40 watts draws 0.16 amps ........ surely it cannot be more efficient to run a 240 volt appliance through an inverter , can it ? please if any one can help here it would be much appreciated .

[s/v Faith]

Good post Dan,

  The only thing I would add is that the inverters are even less efficent as you get into higher ratings.  If you are using a 1000w inverter to charge your cell phone you may be at something like 10% efficient.  I have 2 inverters I use aboard Faith, a 60w inverter and a 600w inverter.  I can run most chargers on the 60w, but the laptop needs the bigger one. 

While higher voltage appliances may be a bit more efficient, since a boat generally only has a 12 VDC power system, it really isn't more efficient to run an appliance through an inverter, especially if you can run the appliance via 12 VDC directly.  The inverter generates heat, which is due to the fact that it is less than 100% efficient. The appliance, especially if it is a 12 VDC appliance to begin with, will also generate some heat in the voltage conversion process—wall wart, transformer, whatever.

As an example, if an appliance, say an Engel dual-voltage refrigerator, draws 3 amps at 12 VDC, it is a 36 WATT appliance.  If you were to run the same refrigerator at 120 VAC, it would only draw about .3 amps, since it is a 36 Watt Appliance (120 VAC * .3 Amps=36 watts).  However, the inverter would only be about 85% efficient, so the actual DC usage of the 36 watt appliance via the inverter would be 42.35 watts of power... (42.35 *.85 = 36 watts).

So, if you run the refrigerator off of 12 VDC, it consumes 36 watts per hour or uses 3 amp-hours per hour of use.  If you run it via the inverter, it consumes 42.35 watts or 3.53 amp-hours per hour. 

OK I have been having a bit of trouble with some info on current draw and the like. A prime example is according to ohms law a 12 volt appliance that draws 12 watts draws 1 amp . a 240 volt appliance that draws 40 watts draws 0.16 amps ........ surely it cannot be more efficient to run a 240 volt appliance through an inverter , can it ? please if any one can help here it would be much appreciated .

[AdriftAtSea]

Very true... sizing the inverter to match the load is your best bet... if you have a 50 watt load, don't use a 1000 watt inverter. :)

[matt195583]

thank you very much for clarifying that on for me  ;D

[AdriftAtSea]

You're very welcome... no charge. :) :D

thank you very much for clarifying that on for me  ;D

[newt]

Used to be that most laptops had a 12 volt charger cigarette plug in, Is that not the case anymore? I am trying to get all 12 volt accessories for my boat because of the reasons outlined above.
Mark

[s/v Faith]

Used to be that most laptops had a 12 volt charger cigarette plug in, Is that not the case anymore? I am trying to get all 12 volt accessories for my boat because of the reasons outlined above.
Mark

You can still buy them, but most laptop's I have come across have been 19.2-20vdc+ for some time now. 

[psyche]

I purchased the Asus mini with 160 gig hard drive with blue tooth (larger of their mini) and love it. I am not bothered by smaller screen and slightly smaller keyboard. You can order in windows XP or linux. It operates on 12 volt current and runs 6 hours on the battery. You can go to the ssca website and read a lot of posts about the Asus and also about which monitors operate on 12 volt current. Some of the appilances in 12 volt do not operate as well as the 120 and the motors don't have the power. I sometimes run power tools on my inverter and can do so with no problem. I have an inverter (1000 watt) installed and a small portable 350 watt that plugs into a cigarette lighter. I am running 4 golf carts but when I add the water maker I may go to 6 golf carts. I have a 75 watt solar panel but will add a 120 over the Bimini. I also have all LED lights in the interior and navigation to conserve. I see power use as a total plan to conserve. I sail a 8.7 Columbia.

[psyche]

Below is a very informatiove pdf book that compares and contrasts various alternative energy systems as well as teachs in fairly plain language the rationale of the systems/theories. enjoy...Dan


http://www.victronenergy.nl/upload/documents/Book-EN-EnergyUnlimited.pdf